European Patent Application No. EP-A-0 114 085 discloses a dimensionally stable anode for an aluminum production cell comprising a conductive substrate of a ceramic, a metal or other materials which is coated with a layer of a fluorine-containing cerium oxycompound called "cerium oxyfluoride". The anode is essentially stable under conditions found in an aluminum production cell, provided a sufficient content of cerium species is maintained in the electrolyte.
The anode described in the above European patent application performs well in respect of dimensional stability. However, contamination of the produced aluminum by substrate components may occur under certain circumstances. As shown by microphotographs, the cerium-containing coating may have a structure with small imperfections such as pin-holes or cracks which produce small interstices between coated areas, allowing access of the electrolyte to the substrate. In such cases, the electrolyte may corrode the substrate leading to a limited but undesired contamination of the aluminum by substrate components.
The above reference also mentions that the protective coating on the anode may consist of the fluorine-containing cerium oxycompound and at least one other material which remains stable at the anode surface and forms a permanent component of the coating during operation. Materials which improve the electronic conductivity or electrocatalytic characteristics of the coating will be preferred.
European Patent Application EP-A-0 203 884 published on Dec. 3, 1986 proposed the addition of yttrium, lanthanum, praseodymium or other rare earth metals to the electrolyte in addition to cerium in order to obtain a cerium oxyfluoride coating which is doped with one of these metals and has an improved microstructure, substantially free of imperfections.
Other techniques have been proposed to preserve coatings eg of TiB.sub.2 on a substrate which is immersed in a solution, by maintaining saturation amounts of titanium and boron in the solution, thus, providing an equilibrium between dissolution and re-deposition of these substances. These methods provide stabilization of the coatings rather than improvement of their morphology.
European Patent Application No. 87200587.1, as yet unpublished, discloses a method of producing a coating or a self-sustaining body comprising cerium oxyfluroide of the formula CeO.sub.x F.sub.y where x=1.5 to 1.99 and y=0.01 to 0.5, by preparing a particulate starting material of the given composition and consolidating it into a shape or into a coating on a substrate. One method was to strip an electrolytically formed coating and then reconsolidate it. Another method was to provide a particulate starting material by reaction sintering.